This article was written by UMN Extension educator Angie Peltier, UMN Extension IPM specialist Bruce Potter, UMN researcher and assistant IPM coordinator Eric Burkness and UMN Extension entomologist Bill Hutchison. Former UMN Extension educator Phillip Glogoza contributed to this survey in 2017 and 2018 and this work could also not have been carried out without the assistance of cooperating NW MN farmers and crop advisors.
Purpose of the study
European corn borer (ECB) larvae tunnel into stalks and ear shanks (Figure 1). Feeding affects the transfer of water and nutrients within the plant and can directly affect yield by reducing kernel weight and number. ECB feeding can indirectly affect yield when tunnels cause stalk breakage, ear drop, or allow the entry of stalk rot and ear mold fungi.
ECB and Bt corn
More than 25 years ago scientists found a way to transfer a gene from a soil-borne bacterium called Bacillus thuringiensis (Bt) into the corn genome. Bt corn was approved for commercial use in 1996. Within the corn plant tissues, this gene produces a protein toxic to corn borer larvae. When ingested by larvae, the protein breaks down to a toxin which kills larvae by allowing mid-gut contents to leak into the rest of the body cavity. Additional Bt traits that target different above- and below-ground insect pests have since been incorporated into some hybrids.
The only way to manage ECB before Bt corn was developed, was to closely monitor ECB moth flights and scout for larvae and egg laying. If ECB populations warrant, foliar insecticide applications can provide control if they are carefully timed as the larvae are only susceptible to insecticides for 10 to 14 days. After that time, 3rd instar larvae begin to tunnel into the stalk or ear shank where they are protected from insecticide applications. This timing can be difficult particularly in areas of the state where both a single generation and multiple generation biotypes of ECB exist. Historically, the single generation (univoltine strain) has predominated in NW Minnesota.
Even the best-timed insecticide application is less effective than growing a hybrid with the Bt trait. Depending on the hybrid and trait package Bt corn can cost up to $20/acre more than conventional seed. In the current economic environment, $20 is a big deal and is a major driver of non-Bt corn hybrid seed purchases. In much of MN, ~80% of corn acres continue to be planted to hybrids with above ground Bt traits. This has resulted in area-wide suppression of ECB populations, protecting even the non-Bt acres.
Study Objectives
Some objectives of the MN Corn Research & Promotion Council-sponsored 2017-2020 fall ECB survey in NW MN are to answer the following questions:
1) Are there changes in ECB population densities over time?
2) To what extent does the area-wide suppression effect occur in the NW?
3) Have any population shifts taken place? ie. is the Bt trait still effective (Bt-resistant corn borer have been found in eastern Canada but fortunately they are a different strain than occurs in MN) and does ECB in NW MN continue to only produce a single generation of larvae each year? Understanding the number of generations per year is essential for managing ECB in non-Bt corn.
Results
During 2017, 2018, 2019 and 2020, a total of 29, 40, 55 and 28 commercial fields were surveyed in NW MN, respectively (Figure 2, Table 1). Among the randomly surveyed fields there were also 11 known non-Bt fields in 2017, 25 in 2018, 36 in 2019 and 8 in 2020. The data presented in Table 1 summarize the per plant average number of ECB larvae in surveyed fields by year and Bt status. In 1995, before the 1996 release of ECB Bt hybrids, an average of 1.16 ECB larvae per plant were found in NW MN corn plants. In 2017 through 2019, randomly surveyed corn fields had an average of 0 to 0.0167 larvae per plant, while the average number of larvae per plant in non-Bt corn fields ranged from 0.0727 to 0.1772 larvae per plant. When compared to randomly surveyed fields, in 2017 there were more than 4.4 times the number of larvae per plant in the non-Bt fields; similarly, when compared to randomly surveyed fields, in 2019 there were 14 times the number of larvae per plant in the non-Bt fields.
ECB population densities were very low in all surveyed fields in 2020. This may indicate that, even at though overall ECB populations are low, they still follow the historical cycle entomologists believe related to a fungal disease and other parasites causing dramatic declines in high ECB populations every 6-7 years.
While higher than number of larvae per plant in fields surveyed at random, the average number of larvae per plant in non-Bt fields is much lower than the traditional economic thresholds levels for ECB (typically estimated at 0.5 larvae/plant).
Table 1. NW MN crop reporting district data for ECB larvae in field corn, Minnesota 2017-20. Baseline data for 1995, prior to Bt corn commercialization is also shown (non-Bt fields)*
Bottom line
While this information provides a ‘30,000 ft view’ of ECB in the region, remember that these are region-wide averages and do not replace scouting of individual fields for making informed, in-season pest management decisions. One positive for those planting non-Bt corn in NW MN, the larvae collected in this region reflect the single-generation type of ECB, meaning that scouting and insecticide management can be confined to a shorter time each year.
Each farmer has a different tolerance for risk. While low populations mean that there is less risk associated with planting corn hybrids without Bt for ECB protection, the risk is not zero.
Want to learn more?
For additional information about the European corn borer and ECB management, visit:
https://extension.umn.edu/corn-pest-management/european-corn-borer-minnesota-field-corn
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